In an apparatus configured to form an electrophotographic image, a heat fixing device including a heating member and a pressure member that is arranged so as to face the heating member is used as a heat fixing device configured to fix an unfixed toner image formed on a recording material to the recording material. The heat fixing device is a device configured to convey the recording material by the rotation of both the members while toner is fixed to the recording material by heat from the heating member and pressure due to pressure contact between both the members.
The pressure member includes a base configured to impart stiffness that withstands pressure contact with the heating member to the pressure member, an elastic layer configured to impart elasticity necessary for the formation of a nip portion to the pressure member, and a surface layer which is composed of a fluorocarbon resin in order to impart toner releasability to the pressure member.
To reduce the power consumption of the heat fixing device, it is desirable to reduce the time required to increase the temperature of the nip portion to a temperature required to fix toner (hereinafter, also referred to as a “warm-up time”). Thus, in the pressure member, the coefficient of thermal conductivity of the elastic layer is reduced by allowing the elastic layer of the pressure member to contain pores. That is, heat conduction through the pressure member is reduced to inhibit the dissipation of heat from the heating member to the base, thereby improving the rate of temperature increase of the heating member.
Here, the following three typical methods are known as methods for producing porous elastic layers having pores. In PTL 1, a foaming agent is mixed with an uncrosslinked silicone rubber. Then the resulting mixture is cured by heating while the mixture is foamed. In PTL 2, a hollow filler is mixed with an uncrosslinked silicone rubber in advance. The resulting mixture is subjected to crosslinking and forming, thereby forming pores. In PTL 3, a water-absorbing polymer that has absorbed water is dispersed in an uncrosslinked silicon rubber. After the crosslinking of the silicone rubber, dehydration is performed to form pores.
The pressure member is required to have improved durability in addition to the reduction in warm-up time described above. In the case where the heat fixing device is used over long periods of time, a wrinkle extending in the circumferential direction of the pressure member (hereinafter, also referred to simply as a “circumferential direction”) can be formed on a surface of the pressure member. In the case where the wrinkle extending in the circumferential direction is formed, a defect can occur in an image portion corresponding to the wrinkle when a different size electrophotographic image is formed.
To inhibit the occurrence of such a wrinkle extending in the circumferential direction, in general, a method is employed in which in the production process of the pressure member, the surface layer is fixed on the elastic layer in a state of being stretched in the longitudinal direction (axial direction) of the pressure member. This suppresses the looseness of the surface layer in the longitudinal direction.
PTL 4 discloses a fixing member in which a fluorine-containing resin tube is fixed on an elastic layer using an adhesive layer with the fluorine-containing resin tube in a state of being stretched in the longitudinal direction.